We use PyBox, a new numerical implementation of the box-model approach, to reproduce pyroclastic density current (PDC) deposits from the Somma–Vesuvius volcano (Italy). Our simplified model assumes inertial flow front dynamics and mass deposition equations and axisymmetric conditions inside circular sectors. Tephra volume and density and total grain size distribution of EU3pf and EU4b/c, two well-studied PDC units from different phases of the 79 CE Pompeii eruption, are used as input parameters. Such units correspond to the deposits from variably dilute, turbulent PDCs. We perform a quantitative comparison and uncertainty quantification of numerical model outputs with respect to the observed data of unit thickness, inundation areas and grain size distribution as a function of the radial distance to the source. The simulations consider (i) polydisperse conditions, given by the total grain size distribution of the deposit, or monodisperse conditions, given by the mean Sauter diameter of the deposit; (ii) axisymmetric collapses either covering the whole 360^∘ (round angle) or divided into two circular sectors. We obtain a range of plausible initial volume concentrations of solid particles from 2.5 % to 6 %, depending on the unit and the circular sector. Optimal modelling results of flow extent and deposit thickness are reached on the EU4b/c unit in a polydisperse and sectorialized situation, indicating that using total grain size distribution and particle densities as close as possible to the real conditions significantly improves the performance of the PyBox code. The study findings suggest that the simplified box-model approach has promise for applications in constraining the plausible range of the input parameters of more computationally expensive models. This could be done due to the relatively fast computational time of the PyBox code, which allows the exploration of the physical space of the input parameters.

中文翻译：

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使用盒模型法再现索马斯维苏威火山79 CE爆发的火山碎屑密度流沉积

我们使用PyBox（盒模型方法的新数值实现）从索马-维苏威火山（意大利）再现火山碎屑密度流（PDC）沉积物。我们的简化模型假设惯性流锋动力学和质量沉积方程式以及圆形扇形内部的轴对称条件。输入参数是EU3pf和EU4b / c的Tephra体积和密度以及总晶粒尺寸分布（来自79 CE庞贝古城喷发不同阶段的两个经过充分研究的PDC单元）。这些单位对应于可变稀薄的湍流PDC的沉积物。我们根据观察到的单位厚度，淹没面积和晶粒尺寸分布的数据，对到源的径向距离进行数值模型输出的定量比较和不确定性量化。模拟考虑（i）由沉积物的总晶粒尺寸分布给出的多分散条件，或由沉积物的平均Sauter直径给出的单分散条件；（ii）轴对称塌陷覆盖整个360^∘（圆角）或分为两个圆形扇区。我们获得的固体颗粒初始体积浓度范围从2.5％到6％不等，具体取决于单位和圆形部分。在多分散和扇区化的情况下，在EU4b / c装置上达到了流量范围和沉积物厚度的最佳建模结果，表明使用总粒度分布和颗粒密度尽可能接近实际条件可以显着提高PyBox代码的性能。研究结果表明，简化的盒模型方法有望在限制更昂贵的计算模型的输入参数的合理范围内得到应用。可以这样做是因为PyBox代码的计算时间相对较快，